Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s. A high voltage discharge between two electrodes excites a laser gas to produce a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam. Many of these electric discharge gas lasers are operated in a pulse mode.
Excimer lasers are a particular type of electric discharge gas laser and they have been known since the mid 1970s. A description of an excimer laser, useful for integrated circuit lithography, is described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled xe2x80x9cCompact Excimer Laser.xe2x80x9d This patent has been assigned to Applicants"" employer, and the patent is hereby incorporated herein by reference. The excimer laser described in Patent ""884 is a high repetition rate pulse laser. These excimer lasers, when used for integrated circuit lithography, are typically operated in an integrated circuit fabrication line xe2x80x9caround-the-clockxe2x80x9d producing many thousands of valuable integrated circuits per hour; therefore, down-time can be very expensive. For this reason most of the components are organized into modules which can be replaced within a few minutes. Excimer lasers used for lithography typically must have its output beam reduced in bandwidth to a fraction of a picometer. This xe2x80x9cline-narrowingxe2x80x9d is typically accomplished in a line narrowing module (called a xe2x80x9cline narrowing packagexe2x80x9d or xe2x80x9cLNPxe2x80x9d for KrF and ArF lasers) which forms the back of the laser""s resonant cavity (A line selection unit xe2x80x9cLSUxe2x80x9d is used for selecting a narrow spectral band in the F2 laser.) The LNP is comprised of delicate optical elements including prisms, a mirror and a grating. Electric discharge gas lasers of the type described in Patent ""884 utilize an electric pulse power system to produce the electrical discharges, between the two elongated electrodes. In such prior art systems, a direct current power supply charges a capacitor bank called a xe2x80x9ccharging capacitorxe2x80x9d or xe2x80x9cC0xe2x80x9d to a predetermined and controlled voltage called the xe2x80x9ccharging voltagexe2x80x9d for each pulse. The magnitude of this charging voltage may be in the range of about 500 to 1000 volts in these prior art units. After C0 has been charged to the predetermined voltage, a solid state switch is closed allowing the electrical energy stored on C0 to ring very quickly through a series of magnetic compression circuits and a voltage transformer to produce high voltage electrical potential in the range of about 16,000 volts (or greater) across the electrodes which produce the discharges which lasts about 20 to 50 ns.
Excimer lasers such as described in the ""884 patent have during the period 1989 to 2001 become the primary light source for integrated circuit lithography. More than 1000 of these lasers are currently in use in the most modem integrated circuit fabrication plants. Almost all of these lasers have the basic design features described in the ""884 patent. This is:
(1) a single, pulse power system for providing electrical pulses across the electrodes at pulse rates of about 100 to 2500 pulses per second;
(2) a single resonant cavity comprised of a partially reflecting mirror-type output coupler and a line narrowing unit consisting of a prism beam expander, a tuning mirror and a grating;
(3) a single discharge chamber containing a laser gas (either krypton, fluorine and neon for KrF lasers or argon, fluorine and neon for ArF lasers), two elongated electrodes and a tangential fan for circulating the laser gas between the two electrodes fast enough to clear the discharge region between pulses of debris from the previous pulse, and
(4) a beam monitor for monitoring pulse energy, wavelength and bandwidth of output pulses with a feedback control system for controlling pulse energy, energy dose and wavelength on a pulse-to-pulse basis.
During the 1989-2001 period, output power of these lasers has increased gradually and beam quality specifications for pulse energy stability, wavelength stability and bandwidth have become increasingly tighter. Operating parameters for a popular lithography laser model used widely in integrated circuit fabrication include pulse energy at 8 mJ, pulse rate at 2,500 pulses per second (providing an average beam power of up to about 20 watts), bandwidth at about 0.5 pm full width half maximum (FWHM) and pulse energy stability at +/xe2x88x920.35%.
When these gas discharge are used as light sources for integrated circuit fabrication they are usually operated in what is known as xe2x80x9cburst modexe2x80x9d operation. For example, a laser may be operated at a repetition rate of 2,500 Hz for 0.3 seconds with pulse energies of about 8 mJ in order to scan a die spot on a silicon wafer. The laser is then xe2x80x9coffxe2x80x9d for a period of about 0.3 seconds while the scanner positions the wafer and the scanner optics for illumination of the next die spot. This routine continues until all of the die spots on the wafer (for example, 200 die spots) have been illuminated. Then the scanner equipment replaces the scanned wafer with another wafer. Thus, the typical laser operating cycle would be:
(1) on 0.3 second
(2) off 0.3 second
(3) repeat steps (1) and (2) 200 times
(4) off 10 seconds
(5) repeat steps (1)-(4) continuously.
This type of operation may be continuous 24 hours per day, 7 days per week with short down times for maintenance or other events.
It is very important that each die spot receive the desired quantity of laser illumination and that the illumination be applied uniformly. Therefore the common practice is to monitor and control the pulse energy of each and every pulse to within a few percent (typically about 6 percent) of a target value (for example, 8 mJxc2x10.5 mJ). Since there are these variations in the pulse to pulse energies, a common practice is to monitor the accumulated energy (referred to as dose energy) in a series of pulses (such as moving window of 30 pulses). These control techniques require the monitoring of the pulse energy for every pulse, utilization of information thus obtained to calculate control parameters for subsequent pulses and the adjustment discharge voltages on a pulse to pulse basis so that both pulse energy and dose are maintained within desired ranges.
Modem integrated circuit fabrication requires the printing of circuits with precise dimensions with accuracies in the range of about 0.5 to 0.25 micron or less. This requires very precise focusing of the light from the lithography light sources through projection optics of the stepper machines. Such precise focusing requires control of the center wavelength and bandwidth of the light source. Therefore, the wavelengths and bandwidth of the laser beam from there laser are typically monitored for each pulse and to assure that they remain within desired target ranges. Typically, the wavelength is controlled using a feedback control based on the monitored values of center wavelength. This feedback signal is used to position the pivoting mirror in the LNP described above to change the direction at which laser light is reflected from defraction grating also in the LNP. The centerline wavelength is monitored on a pulse-to-pulse basis and the wavelength is feedback controlled on as close to a pulse-to-pulse basis as feasible. The response time for center wavelength control of prior art lithographic lasers has been a few milliseconds. Bandwidth is monitored on a pulse-to-pulse basis. Bandwidth can be affected by F2 concentration and gas pressure; so these parameters are controlled to help assure that bandwidth values remain within desired ranges. Prior art lithography lasers typically do not provide for fast response control of bandwidth.
A well-known technique for reducing the bandwidth of gas discharge laser systems (including excimer laser systems) involves the injection of a narrow band xe2x80x9cseedxe2x80x9d beam into a gain medium. In some of these systems a laser producing the seed beam called a xe2x80x9cmaster oscillatorxe2x80x9d is designed to provide a very narrow bandwidth beam in a first gain medium, and that beam is used as a seed beam in a second gain medium. If the second gain medium functions as a power amplifier, the system is referred to as a master oscillator, power amplifier (MOPA) system. If the second gain medium itself has a resonance cavity (in which laser oscillations take place), the system is referred to as an injection seeded oscillator (ISO) system or a master oscillator, power oscillator (MOPO) system in which case the seed laser is called the master oscillator and the downstream system is called the power oscillator. Laser systems comprised of two separate systems tend to be substantially more expensive, larger and more complicated to build and operate than comparable single chamber laser systems. Therefore, commercial application of these two chamber laser systems has been limited.
What is needed is a better control system for a pulse gas discharge laser for operation at repetition rates in the range of about 4,000 to 6,000 pulses per second or greater.
The present invention provides a control system for a modular high repetition rate two discharge chamber ultraviolet gas discharge laser. In preferred embodiments, the laser is a production line machine with a master oscillator producing a very narrow band seed beam which is amplified in the second discharge chamber. Feedback timing control techniques are provided for controlling the relative timing of the discharges in the two chambers with an accuracy in the range of about 2 to 5 billionths of a second even in burst mode operation. This MOPA system is capable of output pulse energies approximately double the comparable single chamber laser system with greatly improved beam quality.
In preferred embodiments a single very fast response resonant charger charges in parallel (in less than 250 microseconds) separate charging capacitors for the master oscillator (MO) and the power amplifier (PA). Preferably the charger includes a De-Queuing circuit and a bleed down circuit for precise control of charging voltage. In this embodiment a fast response trigger timing module provides a trigger signal and monitors light-out signals with better than nanosecond precision. In a preferred embodiment a control processor is programmed with an algorithm for generating small charging voltage dithers to produce feedback responses from which trigger timing can be controlled to maintain laser operation within desired ranges of laser efficiency and/or beam quality. In preferred embodiments the system may be operated as a KrF, an ArF or as an F2 laser system. Pulse power components are preferably water cooled to minimize heating effects. The MO may be operated at a reduced gas pressure or lower F2 concentration as compared to the PA for narrower bandwidth. Also, the MO beam is apertured significantly to improve beam spectral quality. Trigger timing techniques are also disclosed to produce improvements in beam quality. In addition, an improved line narrowing module also contributes to better beam spectral quality. In a described preferred embodiment, the laser system includes a control area network (CAN) with three CAN clusters providing two-way communication from a laser control platform to various laser modules. Preferred embodiment of the laser system also include a pulse stretcher for increasing the length of laser pulses and a beam delivery unit with control over beam alignment.